Photopolymers have found widespread application in the preparation of printed circuit boards and other electronic devices. These materials are applied to circuit boards by screen printing and/or stenciling techniques which are well known in the art. Applicant's prior U.S. Pat. No. 4,404,903 describes a semi-automatic device that uses a horizontal print stroke to print a photoimagable soldermask ink on one side of a board. Applicant's prior U.S. Pat. No. 4,957,044 described an automatic device that uses vertical flood and print strokes to print photoimagable soldermask ink simultaneously on both sides of a board. Typically, a single flood stroke is used with a repeated print stroke. There are numerous advantages to simultaneously printing both sides of the board including: the avoidance of many individual handling steps; a single baking step that avoids the undercure of one side and overcure of the other when two separate printing steps are used; the prevention of oxidation of the copper which can occur when one side is uncoated during baking; the prevention of contamination when a board is handled with one side coated before the other; and elimination of the side-to-side variation inherent with single-sided coaters. In addition to overcoming these disadvantages, the vertical screening apparatus of U.S. Pat. No. 4,957,044 also provided a mechanism for avoiding the clogging of holes in the circuit boards with the soldermask ink. This was accomplished by jogging the chase holding the screens between each sequence of flood and print strokes.
The apparatus of U.S. Pat. No. 4,957,044 accomplished its purposes by means of a board transport mechanism which sequentially transported the boards into one side of a print zone of the assembly and, after the flood and print sequence, removed it from the other side of the print zone to carry it to a baking station. Each circuit board was secured at its top edge and carried, essentially hanging, through the print zone. A drawback to this arrangement, however, is that the apparatus is rather large, occupying a substantial amount of floor space to permit the travel from one side of the print zone to the other side and then through to the baking oven. In addition, while the mechanism of chase jogging between flood and print sequences went a long way toward eliminating the build-up of soldermask ink in the circuit board holes, it did little to eliminate another problem related to the common presence of corners defined by the meeting of vertical and horizontal circuit lines on the board. Strictly vertical print sweeps can, and often do, leave small voids at such corners.